1. Field of the Invention
This invention relates to an device and a process for the extracorporeal purification of blood and plasma. Specifically, it relates to a purification device which, while similar in structure to hemodialysis devices used in the treatment of renal insufficiencies, uses novel biological means to perform many of the functions of a normal human liver. The device and method are therefore intended to assist in the treatment and support of patients suffering from liver disease or who have undergone transplantation of liver tissue.
2. History of the Prior Art
To best understand the invention, an overview of principles of anatomy and physiology relating to human liver function and disease is useful. The liver is an organ divided into two principal lobes made up of functional units called lobules. A lobule consists of cords of hepatic cells arranged radially around a central vein. Between these cords are sinusoid spaces lined with phagocytic cells known as Kupffer cells. Oxygenated blood is provided to the liver via the hepatic artery while deoxygenated blood leaves the liver via the hepatic portal vein. Branches of these vessels deliver blood to the sinusoids, where oxygen, most nutrients and certain toxins are extracted into the hepatic cells.
More specifically, as glucose-rich blood passes through the liver, excess glucose is removed and stored as the polysaccharide glycogen. When the level of glucose in the blood drops below normal, glycogen will be broken down into glucose which is released by the hepatic cells into the blood stream. The liver also assists protein metabolism by extracting and storing excess amino acids in the bloodstream for use in the construction of many plasma proteins, such as albumin. Bile, a solution of salts, bilirubin, cholesterol and fatty acids which assists in the emulsification of fats and intestinal absorption of lipids, is also produced by hepatic cells. It is not, however, normally secreted into the bloodstream buy these cells but is instead transported to, and stored in, the gallbladder.
Of greater importance to this invention is not the liver's role in digestion of food but its role in regulating the concentration of wastes and toxins in the blood. Hepatic cells contain enzymes which either break down toxins carried in the blood, transform them into less harmful substances or, failing either of those processes, stores them. For example, metabolism of amino acids will result in the release of free amino acids and nitrogenous wastes, the latter of which are converted by hepatic cells to urea. In moderate amounts, this urea is harmless and is easily excreted by the kidneys and sweat glands. “Old” red blood cells and certain bacterial can also be destroyed and, in the case of the former, recycled by the Kupffer cells.
In short, the liver is vital to maintaining the body's normal biochemical state. Impairment or loss of its function can, therefore, be fatal. A concise summary of known possible derangements of hepatic metabolism can be found in Podolsky, et al., “Derangements of Hepatic Metabolism” Ch. 315, Principles of Internal Medicine, 10th ed., pp. 1773-1779, 1983. The medical art has developed several approaches to the treatment of, or compensation for, liver disease, damage and failure. In addition, humans (as well as many other species) are capable of regenerating lost or damaged liver tissue.
However, although supportive and pharmaceutical treatments or transplantation may alleviate or reverse many symptoms of liver disease, these methods all require time which an actually ill patient may not have. Further, while undergoing treatment, support for any loss of normal liver function must be provided to maintain or approximate metabolic homeostasis. A means, therefore, is needed which can perform the cleaning functions of the liver when it cannot, thus increasing the time available for treatment.
Extracorporeal liver perfusion (i.e., pumping blood through foreign liver tissue) has been a proposed means for treatment and support for many years, with mixed success. An example of the use of repeated liver perfusions for long-term hepatic support can be found in Abouna, et al., “Long-Term Hepatic Support by Intermittent Liver Perfusions”, The Lancet, pp. 391-396, (Aug. 22, 1970), which reports maintenance of a patient suffering from liver failure for 76 days using periodic liver perfusions. However, despite attempts to use liver tissue from 5 different species, immunological and other biochemical reactions limited the use of the perfusions and the patient died before a suitable transplant donor could be found.
Isolated hepatocyte transplantation has also be performed, again with mixed results (see, e.g., Makawo, L., et al., Can. J. Surg., 24:39-44, 1981, and Demetriou, et al., Proc. Natl. Acad. Sci. USA, 83:7475-7479, 1986).
In contrast, extracorporeal methods of purifying blood and plasma; i.e., by hemodialysis, hemoperfusion or hemofiltration are well-known and established in the art for treatment of renal insufficiencies. The major goal of these methods is to maintain fluid and electrolyte balance and rid the body of waste products.
In renal hemodialysis, blood is pumped into a dialyzer containing an artificial semipermeable membrane suspended in a dialysis solution. With a concentration gradient established across the membrane for a particular substance, flow from the blood into the dialysis bath will occur. This method can be used to successfully lower the concentration in blood of urea and in plasma of potassium. Net removal of substances whose concentrations should not be altered in blood or plasma, such as sodium in the latter, can be removed by establishing a hydrostatic pressure gradient across the membrane, creating a convective pathway for movement of solutes across the membrane. Details concerning the structure of a conventional hemodialysis device as well as means for controlling fluid temperature, dialyzate concentration, and fluid flow therein are set forth in several existing patents, including, respectively, U.S. Pat. No. 5,011,607 to Shinzato, U.S. Pat. No. 4,923,598 to Schal, U.S. Pat. No. 4,894,164 to polaschegg, and U.S. Pat. No. 5,091,094 to Veech.
In operation, blood is removed or pumped directly from the patient into the dialyzer and flows along one side of the membrane. The dialysis solution is pumped in counterflow across the membrane; effluent blood is returned to the patient.
Hemodialysis according to the method outlined above is most effective for the removal of small molecular weight species that are water-soluble and not protein-bound. As a result, it is principally used in therapy for renal insufficiencies, although it may be used in the treatment of certain drug overdoses. To use the method effectively to compensate for loss of liver function, however, additional strategies for blood detoxification are required.
To that end, a number of implantable and extracorporeal bioartificial liver devices have been proposed in the art and tested in clinical trials (see, e.g., the review in Nyberg, et al., Am.J.Surg., 166:512-521, 1993). Although the design and operation of such devices have varied widely, they share common elements. For example, the devices typically utilize isolated hepatocytes to metabolize solutes from blood which pass through as well as one or more permeable membranes. Of the devices which utilize isolated hepatocytes, the hepatocytes are typically anchored onto a supporting substrate to facilitate cellular differentiation and aggregation.
For example, using many of the concepts disclosed in the parent application of this continuation-in-part application, a bioartificial liver containing hepatocytes bound to collagen-coated microcarrier beads (CYTODEX 3 beads, a trademarked product of Pharmacia) in a hollow fiber containing bioreactor was tested and described as producing more efficient metabolite transfer than systems which entrap hepatocytes within gel or gel droplets (Rozga, et al., Biotech. and Bioengineering, 43:645-653, 1994; see also, Miura, et al., Artif.Org., 10:460-465, 1986 [re use of a calcium alginate gel as a cell support], and Cai, et al., Artif.Org., 12:383-393, 1988 [microencapsulation of cells in a gel]).
Other approaches to the use of isolated hepatocytes in an artificial liver have attached the cells to microcarriers and placed them into a chromatography column for perfusion (Demetrious, et al., Ann.Surg., 259-271, 1986), onto hollow fibers (Jauregui, et al., J.Cell Biochem., 45:359-365, 1991; see also, U.S. Pat. No. 5,043,260), onto glass plates stacked in a module and perfused with oxygenated medium (Uchino, et al., ASAIO Proc., 34:972-977, 1988), onto asialoglycoprotein polymers (Akaike, et al., Gastroenterol., 28 Supp. 45-52, 1993), and into beds packed with matrix-forming materials such as glass beads (Li, et al., In Vitro Cell Dev. Bio., 29A:249-254, 1993). The efficacy of these approaches has been limited by relatively short periods of cell viability (as short as a few hours; Demetriou, et al.), difficulties in forming cell aggregates, and diminished contact between the cells and nutrients, metabolites and toxins (as a result of immobilization of the cells onto a substrate which masks a portion of the cell surface).
Over time, techniques to improve cell viability and aggregation have been improved (see, e.g., published patent application 93-272876 [WO 9316171], which describes a system similar to that disclosed in Li, et al., In Vitro Cell Dev. Bio., supra). However, it has been generally accepted in the art that hepatocyte aggregation and function sufficient for use in extracorporeal liver support are dependent at least on attachment of the cells to a substrate or matrix, if not also immobilization of the cells (see, e.g., Rotem, et al., Biotech. and Bioengineering, 43:654-660, 1994 [hepatocytes are anchorage dependent cells]; Miura, et al., Biomatter Artif. Cells Artif. Org., 18:549-554, 1990 [hepatocyte aggregation is required for proper cell function; to that end, immobilization of the cells is preferred]; Rozga, et al., Ann. Surg., 217:502-511, 1993 [attachment of cells to microcarriers enhances cell function and differentiation]; and, Rozga, et al., Biotech. and Bioengineering, supra [attachment of cells to microcarriers or entrapment of cells in a gel preferred]). In contrast, conventional hemodialysis utilizing a membrane against a suspension of free (i.e., “unattached”), isolated hepatocytes has not been shown to be clinically effective in providing liver support (see, e.g., Olumide, et al., Surgery, 82:599-606, 1977). Thus, the bioartificial liver devices that utilize isolated hepatocytes which are presently being developed and tested in the art anchor the cells to a substrate, a process which entails a relatively delicate manufacturing step informing the cell/substrate attachment, and risks damage to the cells.